The present invention relates to a low pass filter and in particular to a voltage controlled low pass filter.
Low pass filters are used to pass DC and AC signals at frequencies below a cut off frequency and to attenuate AC signals at a frequency above the cut off frequency. They have many applications and may be constructed in many different ways. Generally, voltage controlled low pass filters comprise components some or all of which are discreet components rather than being on-chip integrated components. This has been considered necessary to achieve adequate control of the relationship between the controlling voltage and the filter cut-off frequency. Clearly it would be desirable to manufacture such filters from components all on a single chip so as to reduce size, power consumption and unit costs but it is known to be difficult to manufacture on-chip components the electrical parameters of which are sufficiently accurately determined to provide predictable performance in an analogue circuit such as a voltage controlled low pass filter.
International patent application number PCT/GB97/02336 the content of which is incorporated herein by reference, describes field programmable analogue devices incorporating functional components from which it is possible to assemble many different circuits including for example a single pole filter, an integrating oscillator, a voltage controlled oscillator and a frequency doubler. The functional components include sub-circuits corresponding functionally to add, negate, log, exp (antilog), differentiate and integrate. A single pole filter can be produced by combining add, integrate and negate circuits such that an input signal to be filtered is applied to an add functional component, the output of that component is applied to the input of an integrator component, and the output of that component provides the filtered output signal which is fed back via the negate component to a second input of the add component. Details of such a circuit have been published in promotional material distributed in July 1997 (the TRAC user guide published by Zetex Plc of Fields New Road, Chadderton, Oldham, OL9 8NP, United Kingdom). That published material also includes a copy of the paper xe2x80x9cA Computational Approach to VLSI Analogue Designxe2x80x9d, D. L. Grundy, Journal of VLSI Signal Processing 8, 53-60 (1994). That paper describes the series connection of log and antilog functional components to demonstrate that the output of such an arrangement is a temperature-independent replica of a bipolar input signal. None of the published material however suggests how a voltage controlled low pass filter can be defined using on-chip components such as those described in the above mentioned international patent application number PCT/GB97/02336.
It is an object of the present invention to provide a voltage controlled low pass filter having predictable electrical characteristics using simple on-chip functional components.
According to the present invention, there is provided a voltage controlled low pass filter comprising first and second circuits connected in series between a filter input and a filter output and a third circuit for adding a DC bias voltage to the filter input with AC signals to be filtered, wherein the first circuit has a gain inversely related to the DC bias voltage, the first circuit is operative to convert any signal applied to the filter input into a first circuit output signal which is a logarithmic function of the applied signal, the second circuit has a gain directly related to the DC bias voltage such that the overall gain of the first and second circuits is unity, the second circuit has a bandwidth which is inversely related to the gain of the second circuit, and the second circuit converts the first circuit output signal into a signal at the filter output which is an exponential function of the first circuit output signal, whereby the filter has a cut off frequency which is dependent upon the DC bias voltage and the filter output signal is a replica of the signal applied to the filter input for frequencies below the cut off frequency.
Given that the second circuit has a bandwidth which is inversely related to the gain of the second circuit, and that gain is directly related to the DC bias voltage, changes in the DC bias voltage cause proportional changes in the cut-off frequency of the low pass filter. Thus, simply by adjusting the DC bias voltage the cut off frequency of the low pass filter can be controlled. The DC bias voltage can be selected such that the filter circuit is non-inverting. As a result, a plurality of filter circuits may be connected in series with each of the filter circuits comprising a first and second circuit, thereby achieving overall cut-off frequency control by the application of a single DC bias voltage to the first filter circuit in the series.
The first circuit may comprise an amplifier having an inverting input connected by a series resistor to the filter input and an output connected to the second circuit, a diode being connected between the non-inverting input and the output of the first circuit amplifier. The second circuit may comprise an amplifier having an inverting input connected by a series diode to the first circuit and an output connected to the filter output, a resistor being connected between the non-inverting input and output of the second circuit amplifier. By appropriate selection of the components making up the first and second circuits, the required overall unity gain can be maintained whilst retaining the ability to adjust the cut-off frequency of the filter by the application of a single DC bias voltage.